Combination, decomposition and displacement

Combination: two or more substances unite into one, A + B → C, and at least one element changes oxidation number: C + O₂ → CO₂ (0 → +4 for C) and 3Mg + N₂ → Mg₃N₂ (0 → −3 for N).

Decomposition: one compound breaks into two or more parts, at least one of them an element: 2H₂O → 2H₂ + O₂, 2NaH → 2Na + H₂ and 2KClO₃ → 2KCl + 3O₂ (Cl +5 → −1, O −2 → 0).

Not every decomposition is redox. CaCO₃ → CaO + CO₂ involves no change in any oxidation number.

Metal displacement: a metal in a compound is replaced by another metal from its elemental state, as in CuSO₄ + Zn → Cu + ZnSO₄. Reducing metals such as Ca, Mg and Al are used this way to win V, Ti and Cr from their oxides or chlorides (V₂O₅ + 5Ca, TiCl₄ + 2Mg, Cr₂O₃ + 2Al).

Hydrogen displacement: alkali metals and some alkaline earth metals (Ca, Sr, Ba) displace hydrogen from cold water; less active metals such as Mg and Fe need steam (2Fe + 3H₂O → Fe₂O₃ + 3H₂).

Many metals, including some that ignore steam (Cd, Sn), release hydrogen from acids. Zn, Mg and Fe react with HCl at rates that show their activity, magnesium fastest and iron slowest; Ag and Au do not react with HCl at all.

Halogen displacement: oxidising power falls from F₂ to I₂. Fluorine even displaces oxygen from water (2H₂O + 2F₂ → 4HF + O₂). Chlorine displaces Br⁻ and I⁻, and bromine displaces I⁻; the freed Br₂ or I₂ is identified by its colour in a CCl₄ layer (Layer Test).

Since no chemical oxidant is stronger than F₂, fluoride ions can be turned into F₂ only by electrolysis.